-74- 



may be vital to the food requirements of many species of 

 seabirds, including some which breed outside the Antarctic 

 region. 



Information on seabird distribution and movements in 

 winter is virtually non-existent although extensive move- 

 ments away from the continent have been reported in East 

 Antarctica (Oordt and Kruyt, 1953) and analogous movements 

 recently noted along the Antarctic Peninsula (Fraser, pers . 

 comm. ) . Laws (1977b) estimated the winter avian biomass 

 to be only about 80% of that in summer. However, since 

 penguins may comprise 90% of this biomass (and most evidence 

 indicates that penguins remain in the Antarctic over winter) , 

 his estimates are most likely incorrect. 



c. Standing Stock and Productivity 



As with penguins, estimating biomass and standing stocks 

 of other seabirds is difficult. The burrowing, nocturnal 

 habits of some species and the remote, inaccessible breeding 

 sites of others make research difficult. Hence, the 

 comparative summary of standing stock and biomass estimates 

 in Table 14 may include underestimates. 



Ecological data on these species are, except for the 

 Diomedeidae and Stercorariidae , rather incomplete. Generally, 

 however, reproductive rates of most species seem low as is 

 the proportion of young reaching breeding age (Carrick and 

 Ingham, 1967). For species on which data are available, the 

 percentage reaching breeding age is often well below 50%, 

 while adult survival is often more than 90% (Pryor, 1967; 

 Ashmole, 1971; Hudson, 1966; Tickell, 1968b, 1970; Tickell 

 and Pinder, 1975; Beck, 1969, 1970). 



d. Food Habits 



Table 15 summarizes the feeding habits of Antarctic sea 

 birds. Though some species such as the blue and snow 

 petrels, Antarctic terns and Wilson's petrels are known to 

 depend heavily on krill, all seabirds may feed on krill at 

 times (Voous , 1965). In some species, krill are key prey 

 items during particular times of the year. Dominican gulls, 

 for example, utilize krill extensively prior to egg-laying, 

 while at other times molluscs are more important food items 

 (Fraser, pers. comm.). Other marine crustaceans (amphipods 

 and isopods) are also important to some diving petrels and 

 southern fulmars, though again seasonality and feeding 

 location are important factors. Ozawa et al . (1968) re- 

 ported that several species of seabirds concentrate to feed 

 on patches of krill (Table 16) , and suggested that, for many 

 species, aggregations almost twice the average density would 



